Which Berlin airport is most convenient to get into the city?
There is pretty good transportation from either airport. Tegel takes less time but a bus is required. Schoenfield has more direct rail connections, but takes a little longer. When Berlin-Branden (BER) is opened (near the current Schoenfield, Tegel will close, and BER will be the only option.
@Lee, I saw a status report on the new Berlin airport a few days ago, and it looks like it's not going to be open anytime soon. There are thousands of defects that need to be remedied, and some of them are significant. Here's a link: www.jaunted.com/story/2013/9/4/15821/17342/travel/Berlin%27s+Delayed+Brandenburg+Airport+Has+an+Incredible+66%2C000+Problems+ However I saw another report indicating that they may try to squeeze EasyJet into one of the smaller Terminals sometime this fall. They did such a great job with the new Hbf, so it's surprising that the airport is such a mess. Cheers!
@George, I hadn't heard about the Hbf needing repairs. That's unfortunate but since it used some unconventional techniques to build on top of a Bog it may not be unexpected. Even so, from what I remember it opened reasonably close to being on-time and on-budget, something the airport certainly won't be doing.
If you're flying from New York City, look into Air Berlin, which goes nonstop from JFK to Tegel. If you are taking a taxi from the airport, Tegel is closer to the center than Schoenefeld, so the taxi will cost less. (If you're not taking a taxi, I'm not sure there's that much of a difference in cost).
United also has direct flights to Berlin Tegel from Newark (EWR) and vice versa.
My Tegel Taxi transfer to Prenzlauer Berg neighborhood was 20 euros on a Saturday. The Taxi Driver complained a couple of times about the traffic. He said "should be like this only week time". I had a horrible experience with a "lost baggage" situation which is best left for another post. After all my previous times in German airports this came to me as a shock. Yes, without question I'd use TXL again else I wouldn't be posting here.